1. Field of the Invention
This invention pertains to the passivation of crystalline substrates used in the fabrication of semiconductor devices, and more particularly to the fluorine passivation of polycrystalline silicon, germanium, gallium arsenide or other III-V compound substrates, and II-VI compound substrates (e.g., cadmium telluride, zinc sulfide, and zinc selenide) used in the fabrication of semiconductor devices.
2. Prior Art
It is well known that dislocations, grain boundaries and the like present in crystalline silicon, germanium, gallium arsenide and other III-V compound and II-VI compound semiconductor materials give rise to minority carrier recombination. It is thought that the so-called bent, dangling, or stretched bonds believed to exist at these crystalline defects give rise to energy levels where minority carriers recombine so as to render the minority carriers electrically inactive. Since the efficiency of a semiconductor device depends, in part, on the number of available charge carriers, it is desirable to reduce the incidence of such recombination.
Bulk hydrogen passivation has been widely employed in the fabrication of semiconductor devices, especially solar cells, made from p-type polycrystalline silicon substrates, as a method of reducing minority carrier recombination losses. For instance, in U.S. Pat. No. 4,557,037, hydrogen passivation is employed in the fabrication of solar cells as a method of improving cell efficiency by reducing recombination. J. I. Hanoka in the article "Hydrogen Passivation of Polycrystalline Silicon", published in NATO ASI Series B: Physics, vol. 136, 1986, pp. 81-90, Plenum Press, New York, describes the effects of hydrogen passivation of a polycrystalline substrate.
Bulk hydrogen passivation has a significant disadvantage when performed in conjunction with a
semiconductor fabrication process involving relatively high temperature, i.e. above 325.degree. C., process steps. In large part because the silicon-hydrogen bond is relatively weak, hydrogen tends to migrate out of the silicon substrate at temperatures above about 325.degree. C. This break up of the silicon-hydrogen bond, which reduces the benefits of passivation, is especially problematic when the substrate is exposed to subsequent higher temperature processing steps.
In part as an attempt to overcome these drawbacks of known hydrogen passivation processes, fluorine has been used as a passivant in amorphous silicon. Fluorine was chosen as a passivant since the bonding energy of the silicon-fluorine bond is up to 60% greater than that of the silicon-hydrogen bond. H. Matsumura, Y. Nakagome and S. Furukawa in the article "A Heat-Resisting New Amorphous Silicon", published in Applied Physics Letters, vol. 36(6), Mar. 15, 1980, pp. 439-440, disclose a method of bulk passivating an amorphous silicon substrate using SiF.sub.4 gas. Fluorine passivation of amorphous silicon substrates is also disclosed in U.S. Pat. Nos. 4,605,941, 4,522,663 and 4,520,380 to Ovshinsky et al, and U.S. Pat. No. 4,569,697 to Tsu et al. C. J. Fang, L. Ley, H. R. Shanks, K. J. Gsuntz, and M. Cardona in the article "Bonding Of Fluorine In Amorphous Hydrogenated Silicon", published in Physical Review B, vol. 22(12), Dec. 15, 1980, pp. 6140-6148, disclose the results of infrared spectra measurement experiments of fluorinated amorphous silicon. The silicon samples were passivated by Fang et al in a conventional rf sputtering system. T. Shimada, Y. Katayama, and S. Horigome in the article "Infrared Spectra Of Amorphous Silicon-Fluorine Alloys Prepared By Sputtering In Fluorosilane-Argon Gas Mixture", published in Japanese Journal of Applied Physics, vol. 19(5), May, 1980, pp. L265-L268, discuss the infrared spectra of fluorine passivated amorphous silicon. The silicon samples were passivated by Shinada et al in the diode-type rf reactive sputtering system. No bulk fluorine passivation of polycrystalline silicon was effected by the methods of the foregoing articles. Nor were Kaufman ion sources used to create the fluorine ion beam.
Surface passivation of p-type crystalline silicon using fluorine as the passivant has also been attempted. B. R. Weinberger, H. W. Deckman, E. Yablonovitch, F. Gmitter, W. Kobasy, and S. Garoff in the article "The Passivation Of Electrically Active Sites On The Surface Of Crystalline Silicon By Fluorination", published in the Journal of Vacuum Science Technology A, vol. 3(3), May/June, 1985, pp. 887-891, disclose a method which allegedly provides surface passivating p- and n-type crystalline silicon substrates by fluorination. This method involves immersing the silicon sample in an aqueous HF solution. In a subsequent article, it was determined that hydrogen surface passivation and not fluorine surface passivation is achieved by the process described in the Weinberger et al article. See, E. Yablonovitch, D. L. Allara, C. C. Change, T. Gmitter, and T. B. Bright, "Unusually Low Surface-Recombination Velocity on Silicon and Germanium Surfaces", Physical Review Letters, vol. 57, no. 2, July 14, 1986, pp. 249-252.
B. R. Weinberger, G. G. Peterson, T. C. Eschrich, and H. A. Kransinski in the article "Surface Chemistry of HF Passivated Silicon: X-ray Photoelectron and Ion Scattering Spectroscopy Results", Journal of Applied Physics, vol. 60 (9), Nov. 1, 1986, pp. 3232-3234, disclose a process for fluorine surface passivating a silicon surface.
Bulk fluorine passivation of crystalline substrates has been attempted using plasmas. D. S. Ginley in the article "Modification Of Grain Boundaries In Polycrystalline Silicon With Fluorine And Oxygen", published in Applied Physics Letter, vol. 39(8), 15 October 1981, pp. 624-626, discloses a method used in an attempt to bulk fluorine passivate a p-type silicon substrate using a dc plasma device. Bulk passivation of the p-type crystalline substrate was not achieved. K. S. Jones and S. J. Pearton in the article "Grain Boundaries In Germanium: Effects Of Exposure To Plasmas", published in Conference Proceedings, from the 13th International Conference of Defects in Semiconductors, pp. K101-K103, describe an unsuccessful attempt to fluorine passivate a germanium substrate.
As the foregoing references indicate, fluorine passivation of amorphous silicon substrates is well known. On the other hand, bulk fluorine passivation of p-type crystalline, especially polycrystalline, silicon and germanium substrates has not been achieved to date.